Dictionaries

Dictionaries 

A dictionary in Python is a collection of key-value pairs. The dictionary is surrounded by curly braces. Each pair is separated by a comma and the key and value are separated by a colon. Here is an example:

state_capitals = { 'Arkansas': 'Little Rock',
                             'Colorado': 'Denver',
                             'California': 'Sacramento',
                             'Georgia': 'Atlanta' }

To get a value, refer to it by its key:
ca_capital = state_capitals['California']

You can also get all of the keys in a dictionary and then iterate over them:

for k in state_capitals.keys():
              print('{} is the capital of {}'.format(state_capitals[k], k))


Dictionaries strongly resemble JSON syntax. The native json module in the Python standard library can be used to convert between JSON and dictionaries.


defaultdict 

A defaultdict is a dictionary with a default value for keys, so that keys for which no value has been explicitly defined can be accessed without errors. defaultdict is especially useful when the values in the dictionary are collections (lists, dicts, etc) in the sense that it does not need to be initialized every time when a new key is used.

A defaultdict will never raise a KeyError. Any key that does not exist gets the default value returned.
For example, consider the following dictionary

>>> state_capitals = { 'Arkansas': 'Little Rock',
                                    'Colorado': 'Denver',
                                    'California': 'Sacramento',
                                    'Georgia': 'Atlanta' }

If we try to access a non-existent key, python returns us an error as follows

>>> state_capitals['Alabama']

Traceback (most recent call last):

 File "<ipython-input-61-23236329695e6f", line 1, in <module>
                     state_capitals['Alabama'] 
KeyError: 'Alabama'

Let us try with a defaultdict. It can be found in the collections module.

>>> from collections import defaultdict
>>> state_capitals = defaultdict(lambda: 'Boston')

What we did here is to set a default value (Boston) in case the give key does not exist. Now populate the dict as before:

>>> state_capitals['Arkansas'] = 'Little Rock'
>>> state_capitals['California'] = 'Sacramento'
>>> state_capitals['Colorado'] = 'Denver'
 >>> state_capitals['Georgia'] = 'Atlanta'

If we try to access the dict with a non-existent key, python will return us the default value i.e. Boston

>>> state_capitals['Alabama']
 'Boston'

and returns the created values for existing key just like a normal dictionary

>>> state_capitals['Arkansas']
'Little Rock'